German Patent DE 25 14 771 C2 describes a heat generator for a household appliance. The steam generator includes an evaporation chamber having fluidically connected thereto a supply line for water and a discharge line for steam, and further includes an evaporation surface that can be heated by a heat accumulator. The steam generator further includes an electric controller which controls or regulates the heating of the heat accumulator by a heater, and the introduction of water by means of a valve located in the supply pipe or a pump. The evaporation surface is provided by the cylindrical circumferential surface of a bore formed in the heat accumulator, said bore forming the evaporation chamber. In this design, a high temperature difference between the temperature of the evaporation surface of the heat accumulator and that of the water to be evaporated leads to what is known as “film boiling” on the hot surface. The resulting steam cushion acts as thermal insulation and prevents effective evaporation.
German Utility Model DE 296 03 713 U1 describes a steam generator having a rotationally symmetric heat accumulator disposed in an evaporation chamber. The evaporation surface is provided by the outer circumferential surface of the heat accumulator. The geometry of the heat accumulator is such that film boiling occurs in one section of the evaporation surface because of the heat transfer conditions occurring therein and that, due to the insulating effect of the steam cushion, heat is conducted in a defined manner to the region of the evaporation surface where nucleate boiling is to be accomplished along with good heat transfer and effective evaporation. In order to achieve this goal, the evaporation surface of the heat accumulator has evaporation ribs formed around its outer surface, the heat flow from the heating element to the evaporation ribs being limited by means provided in the region of the roots of said ribs. In this design, the complex geometric configuration of the heat accumulator is disadvantageous in terms of production costs and the effort required for maintenance, for example, for removal of lime deposits from the evaporation surface.
The heat accumulators described in the aforementioned references have comparatively large masses to obtain a slow and therefore stable steam generator.
As a general principle, it holds that the greater the temperature difference between the accumulator, and thus the evaporation surface, and the water being evaporated, the larger the quantity of water that can be evaporated. The heat transfer rate, and thus the steam generator output, increases. When the temperature difference between the heat accumulator, and thus the evaporation surface, and the water being evaporated is increased above a critical value, the heat transfer rate decreases, goes through a minimum, and then increases again. This is due to the transition from nucleate boiling to film boiling.
EP 1 658 798 A1 describes a thick film heater that uses an approach which explicitly avoids increasing the temperature difference above the critical value, and thus above a critical heat transfer rate.